1. Introduction
This invention relates to microlithography, and more particularly, to a process of microlithography that uses a photoresist coating protected by a metal mask. Specifically, the invention relates to the formation of a patterned metal mask over a photoresist coating, the use of the mask to dry develop the photoresist coating and/or transfer an image defined by a developed photoresist coating to an underlying substrate.
2. Description of the Prior Art
Recent advances in electronic device fabrication have resulted from improvements in manufacturing techniques, especially improvements in microlithographic methods and methods for transferring patterns from a master to a substrate utilizing a patterned photoresist.
The conventional method for transferring an image pattern of actinic radiation to a substrate, such as a wafer used in semiconductor manufacture, comprises coating the substrate with a light sensitive photoresist, exposing the photoresist coating to said patterned radiation to form a latent image in the photoresist coating, developing the photoresist coating using either wet or dry development techniques, and etching or otherwise treating the substrate to transfer the pattern to the substrate.
Wet development of an exposed photoresist coating involves contact of the coating with a developer that preferentially dissolves (develops) exposed or unexposed areas, dependent upon whether the photoresist used is a positive or negative acting photoresist, to form a relief image in the photoresist coating. Dry development most often involves contact of the photoresist coating with an oxygen containing plasma to generate an image by erosion or ablation of the photoresist in a selective pattern.
For fine line patterning, anisotropic dry development with a reactive ion etch is preferred to wet development because anisotropic etching results in a relief image having vertical side walls. This permits transfer of an image of enhanced resolution to an underlying substrate. Following formation of the relief image in the photoresist coating, whether by wet or dry development, the bared underlying substrate may be treated by art recognized processes.
Photoresists used in processes of the above type are organic polymers often containing a separate light sensitive component. Contact of an organic coating with an oxygen plasma causes erosion or ablation of all surfaces of the coating contacted by the plasma stream. For this reason, to create an image pattern in a photoresist coating by dry development with a plasma, the photoresist coating must have areas that are plasma resistant and other areas that are not plasma resistant, the plasma resistant areas and the areas subject to attack by the plasma defining the desired image pattern.
Methods for dry development of photoresist coatings in an image pattern are known in the art. For example, a known method for formation of a plasma resistant photoresist coating in an image pattern involves use of a photoresist containing silicon as part of the photoresist composition in sufficient quantity whereby following exposure to patterned activating radiation and upon contact with a plasma, a silicon oxide mask is formed over the surface of the photoresist, the silicon oxide mask functioning as a patterned protective layer that limits erosion of those portions of the photoresist coating underlying the silicon oxide mask.
A method for generating a pattern in a photoresist coating using a technique similar to that described above is disclosed in U.S. Pat. No. 4,430,153. The method disclosed in this patent involves formation of an etch barrier over a photoresist coating followed by reactive ion etch development. The method comprises coating a surface with a layer of an aromatic polyamic acid; at least partially curing the layer of aromatic polyamic acid to the corresponding aromatic polyimide; in situ converting the surface layer of the aromatic polyimide to a silicon containing alkyl polyamide/imide; applying, exposing and wet developing a layer of photoresist over the silicon containing alkyl polyamide/imide, to selectively expose a portion of the silicon containing alkyl polyamide/imide surface layer; reactive ion etching the exposed portion of the surface layer of the silicon containing alkyl polyamide/imide with carbon tetrafluoride to remove the exposed portion of the silicon containing alkyl polyamide/imide surface layer; and subsequently using the oxygen etch resistant, silicon containing polyamide/imide polymer as a mask in the processing of underlying layers which can be oxygen reactive ion etched.
An additional method for forming a pattern using a technique similar to that described above is disclosed in U.S. Pat. No. 4,426,247. The method comprises the steps of forming a polymer layer on a substrate, forming a silicon layer on the polymer layer, selectively irradiating a surface of the silicon layer with a high energy beam, exposing the surface of the silicon layer to a radical addition polymerizable monomer gas so as to form a graft polymer film on an irradiated portion of the surface of the silicon layer, performing reactive ion etching using the graft polymer film as a mask so as to form a silicon pattern, and a reactive ion etching using the silicon pattern as a mask to protect underlaying organic polymer layers, so as to form an organic polymer pattern.
More recent processes have been developed that permit selective conversion of portions of a nonsilicon containing photoresist to a silicon containing etch resistant mask. In these methods, the nonsilicon containing photoresist is exposed to patterned radiation to create a latent image within the photoresist. The latent image is then reacted with an organo metallic reagent to incorporate an oxide forming element such as a metal or silicon into the image. The metallized or siliconized latent image is then dry developed, and etch resistant images, as well as underlying organic material, if any, can be dry developed using a suitable plasma to sequentially develop and transfer the pattern to a substrate below. Such methods are disclosed in U.S. Pat. No. 4,613,398. As disclosed in said patent, the metallic portion of the organo metallic material can be selected from Group III A metals, Group IV A metals, Group IV B metals, and Group VI B metals. The preferred elemental portions are disclosed as titanium, silicon and tin, with the most preferred being silicon.
The methods used to create dry developable photoresist layers described in U.S. Pat. No. 4,613,398 provide negative patterns. In accordance with the patent, it is also possible to produce a positive pattern. As further disclosed in the patent, several of the methods preferably utilize polymeric materials which initially do not contain a reactive functional group such as hydroxyl, amine, carboxyl, phenol or imide-amine, capable of reacting with an organyl metallic reagent. The reactive functional groups are created within the polymeric material using irradiation or photoactive compounds which are added to the polymeric material and which subsequently react with the polymeric material after exposure to radiation, or combinations thereof. However, these methods encounter difficulties when applied to novolak photoresist materials of the type most commonly used as positive photoresists for microlithography.
Further in accordance with U.S. Pat. No. 4,613,398, to alleviate the difficulties referred to above, and to enable the creation of positive photoresist patterns as well as negative photoresist patterns, an additional method is disclosed that provides for wet development of the latent image created within the photoresist upon exposure to patterned radiation, with subsequent processing to render the photoresist material remaining after development plasma resistant. The plasma resistant photoresist may then be used to transfer the pattern to underlaying polymeric layers.
Additional patents showing dry development of a photoresist coating are shown in the art, for example, a silicon containing photoresist used in a dry development process is disclosed in U.S. Pat. No. 4,433,044. The photoresist of this patent comprises an acrylate and a silicon containing oxime ester of methacrylic acid. A method for selectively dry developing a photoresist coating over a substrate using an oxygen plasma and a photoresist material comprising a poly(silane) sulfone (copolymer) is disclosed in U.S. Pat. No. 4,357,369. A method of producing solid state devices by plasma development of a photoresist film comprising a silicon containing or nonsilicon containing, but organo metallic monomer containing polymer, is described in U.S. Pat. No. 4,396,704.
Though the methods described above yield etch resistant photoresist layers, results are not readily reproducible and the processes are cumbersome, time consuming and costly. Therefore, there is a continuing need for an improved process for providing dry etch resistant masks over photoresists for transfer of micron and submicron images to a substrate.